The Cutting Edge of Japanese Sword 3D Digital Archiving

Introduction: Inheriting 800 Years of Forging Technology in the Digital Age

Japanese swords are far more than mere weapons. They are the crystallization of over 800 years of sword-forging technology, and each intricate detail—such as the hamon, jigane, and nakago—that might escape the naked eye is imbued with the soul of the craftsman, making them works of art. Yet even swords designated as National Treasures or Important Cultural Properties face the constant risk of being lost to fire, earthquakes, or deterioration over time.

Against this backdrop, efforts to record and preserve Japanese swords as precise digital data have rapidly accelerated in recent years. Museums, research institutions, and private companies are collaborating to construct "digital archives" utilizing CT (computed tomography) scanning and high-precision 3D scanners. This article examines the frontline of these efforts from three perspectives: technology, case studies, and challenges.

Recording Sword Blades with CT Scanning and 3D Scanners

CT scanning is a breakthrough technology that enables non-destructive analysis of the internal structure of sword blades. It can visualize in three dimensions the layered structure of tamahagane, the junction between shinzane and kawagane, and the distribution of steel resulting from folding and forging.

The Tokyo National Museum has implemented micro-CT scanning of Important Cultural Property swords, achieving internal structural analysis at resolutions below 0.1 mm. This allows researchers to identify forging methods that are difficult to distinguish from appearance alone—such as the presence or absence of hon sanmai, koubuse, or muneyaki—and to trace the history of past repairs and polishing. The ability to "see" the interior without grinding down the blade is critically important from the perspective of cultural property preservation.

Meanwhile, external measurements using 3D scanners can record the curvature (sori), blade length (hachō), and changes in width at the base and tip with 0.05 mm precision. This offers higher reproducibility than traditional manual measurement and makes it easier to track changes over decades. For sword researchers, this has become a powerful tool for quantitatively comparing the sugata of each sword.

Technical Challenges in Digital Preservation of Hamon

The hamon is the epitome of Japanese sword beauty. It is the white pattern formed during quenching of the blade edge, featuring distinctive designs such as choji, gunome, and suguha, which vary by school and maker. However, digital recording of hamon is technically extremely challenging.

The reason is that the appearance of hamon changes dramatically depending on how light is applied. Under light sources from different angles, it presents entirely different appearances, making it impossible to fully capture its true beauty in a single photograph. The solution garnering attention is RTI (Reflectance Transformation Imaging) technology. It synthesizes multiple images taken under illumination from various angles, allowing the surface condition to be reproduced from any light direction after shooting.

In the "Sword Cultural Property Database" project, conducted jointly by the Agency for Cultural Affairs and the NPO Japan Society of Science for Art, detailed hamon recording using RTI imaging is being attempted. As objective hamon data independent of display lighting conditions accumulates, comparative analysis among researchers becomes significantly easier.

Furthermore, "polarized 3D scanning" technology, which measures glossy metal surfaces with high precision, is approaching practical implementation. While specular reflection interferes with conventional structured light scanners, combining polarized filters allows the subtle texture of the blade surface to be captured. Applications are also expected for recording the texture of jigane—such as itame, mokume, and masame.

Examples of Online Museums and Virtual Exhibitions

The results of digital archiving are increasingly being made available to the public through virtual exhibitions.

At the "TNM & TOPPAN Museum Theater" of the Tokyo National Museum, virtual experiences of swords using VR have been conducted. National Treasure tachi were 3D-scanned and produced as VR content allowing full-scale observation. The experience of freely observing swords from all directions—in contrast to physical exhibitions where viewing is restricted to limited angles through glass cases—was highly acclaimed.

The Kyoto National Museum also participates in the "e-Kokuho" project, making available image data of its Important Cultural Properties and National Treasures. For swords, the collection includes close-up images of the nakago and signature (mei) and photographs of the hamon immediately after polishing, making them valuable research materials.

In the private sector, the specialized sword portal "Touken World" freely publishes a database of thousands of sword images. In addition to allowing cross-institutional searches of art museum and museum collections nationwide, it provides interactive content explaining each part of the sword with 3D models, making it useful as introductory material for beginners.

Looking abroad, the Metropolitan Museum of Art in New York, under its "Open Access" policy, publishes metadata and high-resolution images of its Japanese sword collection under CC0 license. This environment, allowing researchers to freely download and use the materials, significantly supports international sword scholarship.

Current State and Challenges of Digital Technology Applications

While the adoption of digital archiving is steadily advancing, numerous challenges remain to be resolved.

• Lack of Data Standardization: Because each institution records data under its own shooting conditions and formats, cross-institutional comparison is difficult. International standards defining a unified format for sword digital data remain undeveloped, making the development of infrastructure to integrate and utilize data from different institutions an urgent priority.
• Shortage of Specialized Personnel: There are very few individuals possessing both specialized knowledge of 3D scanning and image processing and expertise in sword appraisal. From setting shooting conditions to interpreting acquired data, both technical skill and connoisseurship are essential, and cultivating such personnel requires extensive time and systematic education.
• Cost Barriers: High-precision 3D scanners require equipment investments ranging from millions to tens of millions of yen. The barriers to adoption are high for regional art museums and private collectors, leading to a tendency for digitization to be concentrated in large public institutions. As of 2026, some smaller institutions are beginning efforts utilizing crowdfunding and grants.
• Expectations and Limitations of AI Appraisal: Research using machine learning to estimate maker and school from hamon patterns is progressing. Deep learning models for hamon classification have reported certain levels of accuracy, but appraisal requires multifaceted elements such as visual observation, tactile feedback, and weight balance, and AI has not yet reached the stage of replacing expert experience. Its realistic positioning remains as a "supplementary tool."

Future Prospects: Fusion of Digital Technology and Tradition

Digital archiving brings new possibilities for the "transmission" of Japanese sword culture.

1. Risk Reduction from Loss: Loss due to fire, earthquakes, or theft is the greatest threat to cultural properties, but precise digital data can serve as a guide for restoration. The case of 3D scan data being used to explore restoration of Shuri Castle following its 2019 fire has broadly raised societal awareness of the importance of digital preservation of cultural properties. This lesson has been applied to the sword field as well, accelerating prioritized digitization at major holding institutions.
2. Transmission of Sword-Making Technique: There are expectations to build "digital masters" by precisely analyzing the works of retiring swordsmiths to quantify the characteristics of forging techniques, which the next generation of craftspeople can reference. There is potential to visualize and share part of the techniques traditionally transmitted orally or through observation and imitation in the form of data.
3. Promotion of Global Sword Scholarship: Comparative research between Japanese swords in foreign collections becomes significantly easier through the sharing of digital data. Digital data plays a particularly important role in tracing the provenance of swords dispersed after World War II, and will serve as a key to promoting international cooperation among holding institutions.

Digital archiving of Japanese swords holds the potential to revolutionize all phases—research, education, appreciation, and transmission—beyond mere preservation. From the perspective of the fusion of tradition and cutting-edge technology, the efforts in the sword field are becoming a model case for other cultural heritage disciplines.